Copper carbonate decomposes when heated. The equation for this reaction is: CuCO₃ → CuO + CO₂. Explain why the total mass of the products formed in this reaction equals the mass of the copper carbonate that was heated.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The law of conservation of mass states that atoms are neither created nor destroyed in a chemical reaction. In the decomposition of copper carbonate the equation is
CuCO₃ → CuO + CO₂
and it is balanced, so the numbers of Cu, C and O atoms are the same on both sides. Therefore the total mass of the products (CuO + CO₂) must equal the mass of the reactant (CuCO₃). Even though CO₂ is a gas that leaves the reaction vessel, its mass is still part of the product mass and is therefore included in the total mass of products.
CuCO₃ → CuO + CO₂
and it is balanced, so the numbers of Cu, C and O atoms are the same on both sides. Therefore the total mass of the products (CuO + CO₂) must equal the mass of the reactant (CuCO₃). Even though CO₂ is a gas that leaves the reaction vessel, its mass is still part of the product mass and is therefore included in the total mass of products.
Examiner tips
- Show the balanced equation and state the law of conservation of mass
- Explain that all atoms are present in the products
- Mention that the gas CO₂ still contributes to the product mass
Common mistakes
- Failing to recognise that the equation is balanced
- Ignoring the mass of the released CO₂ gas
- Using vague language such as "mass is conserved" without linking to atoms
Mark scheme (5 marks)
- No atoms are lost or made during a chemical reaction (law of conservation of mass)
- The mass of the products equals the mass of the reactants
- The same atoms that were in the copper carbonate are present in the products (copper oxide and carbon dioxide)
- The equation is balanced, meaning the number of each type of atom is the same on both sides
- Even though CO₂ is a gas that is released, its mass must still be included / accounted for in the total mass of products
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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